Bev, I was having the same problem of the ruler slipping as I reached half way up, then I discovered if I spread my fingers and rest my little finger actually on the cutting mat it stabilises the ruler and alerts me if I am pushing it over as I feel the pressure more on my pinkie....so to recap : spread fingers as wide as possible using thumb as well, position little finger right at the edge of the ruler but on the cutting mat...hope this helps you, it certainly helped me, I like some of the tips here though, and will try a couple out soon :D
Just thought of another thing that you can do, ONLY cut a short area say 6" at a time, double check the ruler then do another short length...double check repositioning your hand further up the ruler as you go too!!